X-ray fluorescence (XRF) spectroscopy is widely recognized as a very accurate method of determining the atomic composition of a material, achieved by irradiating a sample with x-rays and observing the resulting secondary x-rays emitted by the sample.
In general, XRF systems consist of a source of excitation radiation (an x-ray tube or a radioisotope), a means to detect secondary x-rays from the sample and determine their energy or wavelength, and a display of the spectral output. The intensity of the secondary x-rays at certain energies or wavelengths is correlated to the elemental concentration in the sample. Computer software is often used to analyze the data and determine the concentration.
The process begins by irradiating the sample using a source of x-rays. As x-ray photons strike the sample, they knock electrons out of the inner shell of the atoms that make up the sample, creating vacancies that destabilize the atoms. The atoms stabilize when electrons from the outer shell are transferred to the inner shells, and in the process give off characteristic x-ray photons whose energy is the difference between the two binding energies of the corresponding shells. There are two conventional approaches to determining the x-ray spectrum emitted from the sample. The first approach is energy dispersive spectrometry (EDS), and the second is wavelength dispersive spectrometry (WDS). In an energy dispersive spectrometry system, an energy dispersive detector, such as a solid-state detector or a proportional counter, is used to determine the energy spectrum of the emitted photons from the sample. In a wavelength spectrometry system, a crystal or a multi-layer structure is used to select a specific x-ray wavelength from the x-rays photons emitted from the sample.
X-ray fluorescence using EDS is the most widely used method of elemental concentration analysis. This method has some advantages. First, the EDS detector can detect almost all of the elements in the periodic table at once. Second, the system is compact because an additional optic is not required on the collection side compared to wavelength dispersive x-ray fluorescent systems. Third, a low-power x-ray tube may be used because the EDS detector has a large collection solid angle and high efficiency. There are disadvantages to XRF/EDS systems, however, including relatively poor sensitivity and poor energy resolution. Also, because the EDS detector sees all of the x-rays from the sample, the detector is easily saturated by the fluorescent signal from the major elements and the strong scattering of the primary beam.
X-ray fluorescence using WDS has several advantages also, including higher energy resolution and higher signal-to-background ratio compared with XRF/EDS systems. Thus, the XRF/WDS approach is a powerful tool for trace element analysis and applications that require high energy resolution. However, there are disadvantages to conventional XRF/WDS systems, including a requirement for a high power x-ray tube due to limitations of the WDS approach that result in a low efficiency, and a small collection solid angle. Another disadvantage of a conventional WDS system is that the crystal or multi-layer structure on the collection side only selects a specific x-ray wavelength and a scanning mechanism or multi-crystal system is needed for multi-element detection. This has the advantage that detector saturation may be avoided, but it results in a complicated alignment. Therefore, XRF/WDS systems are typically bulky, complex, and more expensive as compared to XRF/EDS systems.
U.S. Pat. No. 5,982,847 to Nelson discloses an energy dispersive (EDS) system, using only polychromatic optics in both the detection and collection paths. No mention is made of diffracting optics in either the excitation or collection paths.
WO02/25258 to X-Ray Optical Systems, Inc. is also strictly an EDS system. Even though monochromatic excitation is used—the detection path is not limited to specific wavelengths with a detection optic—there is no detection optic disclosed or taught by this document. Therefore, the detection system encounters a broader band of wavelengths and processes this broader band using conventional EDS techniques.
EP 0339713 to N. V. Philips discloses a WDS system, however as discussed above, this document discloses the conventional technique of illuminating a very large sample area, a pinhole/slit 6 to define the angle of incidence upon optic 22, thus severely limiting the collection solid angle. There is no disclosure, teaching or suggestion of a focusing optic, providing a small sample spot size, and the attendant advantages of the present invention. The small sample spot size of the present invention is “placed” at position 6, but without limiting the collection solid angle of the detection optic.
Chen, et al, “Microprobe X-Ray Fluorescence with the Use of Point-Focusing Diffractors,” Appl. Phys Lett. 71 (13) 1884-1886, September 1997 is similar to WO02/25258, discussed above. Even though monochromatic excitation is used—the detection path is not limited to specific wavelengths with a detection optic—there is no detection optic disclosed or taught by this document.
U.S. Pat. No. 5,406,609 to Arai et al. is also similar to WO02/25258 —monochromatic excitation with a standard EDS detection scheme.
While most XRF instruments are generally for the analysis of a wide range of elements, there are many important applications in industry process control that require single element or limited element detection. Thus, the present invention is directed to providing compact XRF/WDS systems that provide an ultra high sensitivity or high speed analysis for a limited number of elements.